This invention relates to a method and apparatus for measuring nondestructively, in situ, and in real time, the internal strains from which stresses in composites can be determined. Utilizing an intense pulsed neutron source, a general purpose multi-angle diffractometer and uncomplicated time-of-flight measurements, the invention enables characterization of interfacial bonds in fibers and matrix of metal-matrix and ceramic-matrix composites.
Advanced fiber-reinforced composite materials consisting of a matrix (ceramic or metal, herein called a matrix) and an inclusion (fiber, whisker or particle, herein called a fiber) are being developed for applications in defense, aerospace, energy conservation, and transportation industries. The strength and toughness of fiber-reinforced composites are controlled to a large extent by the nature of bonding between the fibers and the matrix. Ceramic-ceramic composites generally require weak interfacial bonding to achieve high toughness. In certain systems, chemical bonding is supposed to be nonexistent, and frictional forces at the interfaces provide the necessary link between the fibers and the matrix. These frictional forces are dependent on the residual stresses that develop during cooldown after sintering, and it is important to have an idea of the residual stresses that exist in such composites, particularly at the fiber-matrix interface, and of the redistribution of stresses between fiber and matrix due to slippage, cracking or plastic yielding.
It is well known in the prior art that x-ray and neutron diffraction can be used for measuring strain in crystalline metallic or ceramic composites. Change in the interplanar spacing of a set of crystal lattice planes due to strain causes a change in the diffraction angle of the scattered x-ray or neutron beam, and from this change the magnitude of the strain can be determined. Prior art methods using x-ray diffraction are limited, however, because they rely on the relatively weak penetrability of x-rays. X-rays can only measure changes in interplanar spacing at or near the surface of the composite, where the strain that is measured is subject to surface distortion; in most cases x-rays cannot penetrate deep enough to measure bulk strain and thus provide a measure of bulk internal stress at the fiber-matrix interface.
It is also known in the prior art that neutrons can penetrate deeper than x-rays and thus neutron diffraction techniques can provide a bulk or internal measurement. Thermal neutrons are of interest for stress measurements in that they have wavelengths on the order of the lattice spacing, allowing application of Bragg's Law of diffraction to neutrons as: EQU 2d.sub.hkl sin .theta.=.lambda..sub.hkl ( 1)
Where d is the lattice spacing, 2.theta. is the angle between incident and scattered neutron beams when a Bragg peak is detected, .lambda. is the deBroglie wavelength of the neutron, and h, k, and 1 are the Miller indices of the diffracting plane.
As is also well known in the prior art, the determination of stress requires the measurement of strain in at least two spatial directions, and measurement of stress in real-time, for example during applied load or thermal cycling, requires that those two measurements are made simultaneously. With a reactor as a source, a monochromatic neutron beam is used to look in only one spatial direction so that prior art neutron diffraction methods cannot provide the simultaneous measurements of strain in two spatial directions which are necessary for the realtime measurement of stress.
The use of a reactor source in combination with a single angle diffractometer is also very cumbersome and potentially slow because the diffractometer--a bulky, sensitive instrument--must be moved through multiple angles to find the angle at which a Bragg peak is obtained.
In the prior art, a pulsed neutron source which provides a polychromatic beam has been used in combination with a multi-angle diffractometer to detect diffracted beams at multiple angles. However, prior to this invention practitioners have failed to recognize the application to fiber reinforced composites, in that the two instruments in combination enable the simultaneous measurement of strain in the required two directions and the resultant calculation of residual stress.
The present invention uses the General Purpose Powder Diffractometer (GPPD) at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory but is not limited to that equipment. The IPNS produces neutrons by spallation, by bombarding a uranium target with 450 MeV protons. The pulse repetition rate is 30 cycles/s with a peak thermal flux of about 4.times.10.sup.14 neutrons/cm.sup.2. For neutron scattering studies, the fast neutrons are moderated by hydrogenous moderator materials to provide white-beam (polychromatic) slow neutrons.
The GPPD is about 20m from the target. Neutrons are detected with banks of .sup.3 He proportional counters (140 total) encircling the sample chamber on a 1.5m scattered flight path, at 20.degree., 30.degree., .+-.60.degree., .+-.90.degree., and .+-.148.degree. relative to the neutron forward direction. Data are collected by a PDP 11/34-Z8000 computer. With the detectors fixed, the GPPD uses time-of-flight techniques, well known in the prior art, to determine the various wavelength components in a particular scattering experiment.
Prior to the present invention researchers using the IPNS and the GPPD at Argonne National Laboratory have simply applied prior art techniques developed with other equipment configurations and have thus failed to realize the full potential for composites of the powerful combination of the GPPD and IPNS. Until this invention researchers using neutron diffraction to measure stress in an ordered composite have failed to realize that positioning of two detectors as taught in this invention allows the simultaneous measurement of strain in two spatial directions thus reducing the determination of residual stress in the fiber and matrix to acceptable levels of effort.
It is an object of this invention to provide an improved method for the measurement of strain in metal-matrix and ceramic-matrix composites.
It is another object of this invention to provide an improved method for the determination of stress in a stressed sample of an ordered ceramic or metal-matrix composite.
It is another object of this invention to provide a method and apparatus for the measurement of strain which is nondestructive, and effective in air, vacuum or a gaseous environment.
It is another object of this invention to provide a method and apparatus for the measurement of spatial variations of strain within a ceramic or metal-matrix composite.
It is another object of this invention to provide an improved method and apparatus for the measurement of internal strain between a fiber and the matrix in which it is included, enabling nondestructive, in situ, and real time determination of residual stress in a ceramic or metal-matrix composite.
It is another object of this invention to provide an improved method and apparatus for the validation and calibration of other techniques for the measurement of strain and determination of stress (e.g. ultrasonic techniques) and the validation of models used to predict the mechanical behavior of advanced structural composites.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.